In a down link radio section of a long term evolution (LTE) whose specification is being developed in 3rd generation partnership project (3GPP), transmission of radio data from a radio base station to a mobile radio terminal is performed using a radio sub frame having a length of 1 ms as a basic unit.
In this system, a radio sub frame is configured with 14 (or 12) OFDM symbols, a control signal is transmitted using one to three OFDM symbols in the head of each radio sub frame, and a data signal is transmitted using the remaining OFDM symbols.
The control signal includes information used when the mobile radio terminal receives the data signal. Specifically, the control signal includes frequency domain information in which the data signal is arranged, information such as a modulation scheme, a code rate, and a hybrid automatic repeat request (HARQ) which are applied to the data signal, and the like.
FIGS. 1 and 2 are diagrams illustrating exemplary configurations of a transmitting device equipped in a radio base station and a receiving device equipped in a mobile radio terminal according to a related art.
As illustrated in FIG. 1, in a transmitting device 100, a data signal generated by a data signal generator 101 and a control signal generated by a control signal generator 102 are time-multiplexed by a time multiplexer 103, so that a radio sub frame including the data signal and the control signal is configured.
Then, the radio sub frame is subjected to a certain modulation process and a radio transmission process (for example, an up converting process) in a modulating/RF unit 104 and then transmitted to the mobile radio terminal through a transmitting antenna 105.
Meanwhile, in a receiving device 200 of the mobile radio terminal, as illustrated in FIG. 2, a radio signal received through a receiving antenna 201 is subjected to a certain demodulation process and a radio reception process (for example, a down converting process) in an RP/demodulating unit 202 and then time-demultiplexed by a time demultiplexer 203.
A control signal extractor 205 extracts a control signal from an output from the time demultiplexer 203, and outputs the control signal to a data signal extractor 204.
The data signal extractor 204 extracts a data signal from an output from the time demultiplexer 203 based on the control signal output from the control signal extractor 205, and outputs the data signal.
In recent years, mobile communication systems have been continuously developed, and it is preferred to increase a data transmission peak rate and a data transmission throughput while reducing a data transmission delay.
As a technique of reducing a data transmission delay, a technique of reducing a length of the radio sub frame is effective (see FIG. 3A).
However, since a control signal has the size of a fixed value although a length of a radio sub frame is reduced, as a length of a radio sub frame decreases, a ratio at which the control signal occupies in the radio sub frame increases, and a ratio of an area that can be used for transmission of the data signal decreases.
For this reason, as a length of a radio sub frame decreases, data transmission efficiency deteriorates.
Meanwhile, when a length of a radio sub frame is increased, data transmission efficiency is improved, but a delay increases due to data processing such as retransmission control (see FIG. 3B).
In other words, the data transmission efficiency and the data transmission delay have a trade-off relation according to a change in the length of the radio sub frame.
As described above, as mobile communication systems have been continuously developed, opportunities to transmit data of a large size increase, and data that is severe in a required transmission delay characteristic but is not large in the size such as audio data is transmitted as well.
As described above, a preferred transmission rate or a required transmission delay characteristic differs according to data to be transmitted, and thus it is preferable to select an optimal transmission method according to the nature of data.
Thus, it is one of problems to efficiently transmit various kinds of data having different required characteristics in the same radio section.
As a solution approach to solve the problem, a technique of time-multiplexing and transmitting radio sub frames of different lengths on the same carrier has been known (see Patent Reference 1).
Patent Reference 1: Japanese National Publication of International Patent Application No. 2008-535391
FIG. 4 is a diagram illustrating a structure in which radio sub frames of different lengths are time-multiplexed.
When a radio sub frame of a length different from a radio sub frame of a length specified in an existing system is time-multiplexed, a mobile radio terminal designated based on an existing specification can receive only data transmitted through the radio sub frame of the length specified in the existing system.
For example, when a radio sub frame longer than a radio sub frame of an existing specification is newly introduced, a mobile radio terminal designated based on the existing specification is unable to receive data transmitted through the newly introduced long radio sub frame.
In other words, a time zone in which the radio base station is unable to transmit data to the mobile radio terminal designed based on the specification of the existing system occurs, and transmission efficiency of the system deteriorates. For this reason, this approach is inefficient.
As another solution approach to solve the above problem, a technique of transmitting radio sub frames of different lengths on different radio carriers is considered. The radio base station selects a radio carrier to be used to transmit data according to the nature of data to be transmitted. In this case, the mobile radio terminal constantly monitors a plurality of radio carriers in order to be capable of receiving data transmitted through any of the plurality of radio carriers, and thus a processing load of the mobile radio terminal increases.